Explanation
An assumption is something that is believed to be true or certain to happen, without proof. Assumptions are often used to fill in the gaps of incomplete or uncertain information, and they can affect the analysis of the current and future states. In this question, the project manager is assuming that her project team members have a certain skill, without verifying it. This is an example of an assumption that should be identified and validated as part of the assessment of capability gaps.
The other options are not assumptions, but rather:
B: A constraint, which is a restriction or limitation that affects the performance or outcomes of a project or solution.
C: A requirement, which is a condition or capability needed by a stakeholder to solve a problem or achieve an objective.
D: A risk, which is an uncertain event or condition that, if it occurs, has a positive or negative effect on one or more project or solution objectives.
References:
CBAP Handbook, page 14, section 4.1.2.2
BABOK Guide, page 30, section 3.2.1
CBAP / CCBA Certified Business Analysis Study Guide, page 94, section 3.2.1